1. Field of the Invention
The present invention relates generally to a communication system having a unit duplication function, and in particular, to a switching circuit for unit duplication boards, which accommodates both a hardware operation and a software operation for execution and switching of a working mode and a protection mode.
2. Description of the Background Art
Unit duplication boards may be designated as a working board or a protection board by commands generated from a central processing unit (CPU). The central processing unit transmits a working command to the working board and a protection command to the protection board.
The central processing unit senses signals such as a board fail signal and a demount signal and transmits new commands to the working board and protection board to set them to the working or protection mode. Therefore, clock inactivity lasts as long as the response time of the central processing unit, thereby decreasing system stability. If clock inactivity continues, the other peer regards this as loss of frame (LOF) and performs switching, decreasing system stability. Since most clock monitoring circuits use multivibrator chips, clock loss as great as a time constant of a resistor-capacitor circuit (RC) needed for monitoring occurs.
When the communication system is initially operated, a working board is designated by a software command only without using mutual monitoring signals in hardware terms. Without a central processing unit, it is impossible to operate the communication system in software terms. The resulting necessity of hardware operation has introduced alarm signals needed for mutual monitoring such as a demount signal, a signal indicative of the hardware active state of the other board, and signal loss.
Exemplars of the art are U.S. Pat. No. 6,194,969 B1 by Doblar, for System And Method for Providing Master And Slave Phase-aligned Clocks, U.S. Pat. No. 5,521,914 by Mavraganis et al., for Switched Access to Frame Relay, U.S. Pat. No. 5,777,874 by Flood et al., for Programmable Controller Backup System, U.S. Pat. No. 5,627,379 by Bortolini, for Duplication Detection of Circuit Board Removal Using Two Light Emitting Diodes, U.S. Pat. No. 6,144,669 by Williams et al., for Prioritized PVC Management Queues for Improved Frame Processing Capabilities, and U.S. Pat. No. 5,508,910 by Diehl, for Redundant Analog I/O Board System.